1. Field of the Invention
The present invention relates to a pneumatic booster utilized for a brake system of a vehicle or the like.
2. Description of the Prior Art
A pneumatic booster, which is well-known, is provided with a housing partitioned into a constant pressure chamber and a variable pressure chamber by a diaphragm, a power piston attached to the diaphragm, and a valve body attached to the power piston. In the valve body, a plunger is provided. The plunger receives a power generated by the operation of a brake pedal. The communication between the constant pressure chamber and the variable pressure chamber and the communication between the variable pressure chamber and the outside air are respectively controlled according to a relative movement between the valve body and the plunger, and the valve body receives a pressure caused by a differential pressure between the constant pressure chamber and the variable pressure chamber.
In the above well-known pneumatic booster, a stop key is provided to limit a positional relationship between the valve body and the plunger within a predetermined range. The stop key is inserted in a hole formed in the valve body, and a portion of the key fits into a channel formed in the plunger. However, because the hole is formed to have a rectangular cross section, it takes a long time to machine the hole. Also, in case where the hole is formed at the same time as the valve body is molded, there is a problem that a mold for the valve body becomes complicated.